1. Field of the Invention
The present invention relates to an apparatus for learning and controlling an air/fuel ration in an automobile internal combustion engine having an electronically controlled fuel injection apparatus with an air/fuel ratio feedback control function. More specifically, the present invention relates to an apparatus for controlling and learning the air/fuel ratio and then cope with the change of the air density which is due to the altitude.
2. Description of the Related Art
An apparatus for learning and controlling the air/fuel ratio, as disclosed in the specification of U.S. Pat. No. 4,615,319, is adopted in an automobile internal combustion engine having an electronically controlled fuel injection apparatus with an air/fuel ratio feedback control function.
In the control system where a basic fuel injection quantity calculated from a parameter of an engine driving state, which participates in the quantity of air sucked in an engine, is corrected by a feedback correction coefficient set by a proportional-integrating control based on a signal from an air/fuel ratio sensor, such as an O.sub.2 sensor, disposed in the exhaust system of the engine to compute a fuel injection quantity and the air/fuel ratio is feedback-controlled to an aimed air/fuel ratio. According to the above-mentioned conventional technique, the deviation of the feedback correction coefficient from the reference value during the feedback control of the air/fuel ratio is learned for the respective predetermined areas of the engine driving state to determine a learning correction coefficient. In computing the fuel injection quantity, the basic fuel injection quantity is corrected by the learning correction coefficient for each area so that the basic air/fuel ratio obtained by the fuel injection quantity computed without correction by the feedback correction coefficient comes into agreement with the aimed air/fuel ratio, and during the feedback control of the air/fuel ratio, this is further corrected by the feedback correction coefficient to compute the fuel injection quantity.
According to this conventional technique, during the feedback control of the air/fuel ratio, follow-up delay of the feedback control can be prevented at the transient driving, and the desired air/fuel ratio can be precisely obtained at the stoppage of the feedback control of the air/fuel ratio.
Furthermore, a system where the basic fuel injection quantity Tp is determined from the throttle valve opening degree .alpha. and the engine rotation number N, for example. The sucked air flow quantity Q is determined from .alpha. and N by referring to a map and Tp is computed according to the formula of Tp=K.multidot.Q/N(K is a constant). Another system is known where the sucked air flow quantity Q is detected by an air flow meter and the basic fuel injection quantity is computed from the flow quantity Q and the engine rotation number N according to the formula of Tp=K.multidot.Q/N. In the case where a flap type air flow meter (volume flow rate-detecting type) is used as the air flow meter, the change of the density of air is not reflected on the computation of the basic fuel injection quantity, but if the above-mentioned learning control is performed, the computation can cope with the change of the density of air due to the altitude or the temperature of sucked air, so far as learning is advanced in a good condition.
However, in the case where an automobile descend to a lower from a higher land (e.g., a mountain) where the conventional area-wise learning had been advanced, the following problems will occur.
In a deceleration driving of the engine as a transient driving state which often occurs while an automobile is descending, the air-fuel ratio feedback control to supply fuel to the engine is frequently stopped in the deceleration driving state and the fuel supply per se, in general is interrupted under some drifting conditions since the deceleration ability deteriorates due to a response-delay in the air/fuel ratio feedback control and also from the view point of the fuel consumption efficiency. In this situation, accordingly, the air-wise learning control is not carried out at all. Further, since the temperature of the exhaust gas of the engine is low in deceleration driving which is a low-load driving, the O.sub.2 sensor frequently becomes inactive, and the air/fuel ratio feedback control is generally stopped because of the deterioration of is reliability. This also results in the stoppage of the area-wise learning control.
Therefore, even if an acceleration pedal is pressed by chance and the driving enters the other driving region where the area-wise learning control is possible, it is transferred to the deceleration driving before the O.sub.2 sensor becomes active, and the area-wise learning control is also stopped.
Further, even when there are chances to carry out the air-fuel ratio feedback control and the area-wise learning controls in some areas, the number of learning such possible areas is restricted and in the majority of remaining areas, the area-wise learning controls is scarcely advanced.
This description teaches that area-wise learning control is actually rarely performed in descending conditions in a meaningful manner.
As a result, when the injection fuel quantity is computed in the automobile descending based on the area-wise learning correction coefficient which had been learned in the higher altitude, the large deviation of the base air-fuel ratio toward the lean side is produced since the learned area-wise learning correct coefficient cannot respond to the change of the air density which increases with the decrease of the altitude. Appearance of the large deviation of the base air-fuel ratio results in occurrence of troubles such as reduction of the driveability and even stalling of the engine.
When the air-fuel ratio feedback control is restarted immediately after the automobile finishes descending and runs in the lower altitude, since the basic fuel injection quantity is computed based on the area-wise learning correction coefficient which had been learned in the higher altitude, the large deviation of the base air-fuel ratio from the aimed air-fuel ratio toward the lean side due to the control delay results in the same disadvantages as described above.
On the other hand, in the case where the automobile ascends to a higher altitude from a lower altitude, since the ascending driving is a kind of the transient driving, the area for learning is not fixed and even if learning is possible, learning-possible areas are limited while learning is hardly advanced in the majority of areas. Accordingly, in case of the ordinary driving or re-starting of the engine on a flat ground in the vicinity of the summit of the mountain, because of the control delay in the air/fuel ratio feedback control, an over-rich state in the air-fuel mixture gas is produced. This over-rich state is also produced because of the large deviation of the basic air/fuel ratio from the aimed air/fuel ratio at the stoppage of the air/fuel ratio feedback control. Appearance of this over-rich state results in occurrence of troubles such as reduction of the drivability, stalling of the engine and worsening of the re-starting property.
The reason is as follows. Although it is necessary to learn and correct the change of the density of air from the deviation of the feedback correction coefficient from the reference value during the air/fuel ratio feedback control, since the learned deviation includes the deviation of the basic air/fuel ratio which depends on dispersion of parts such as a fuel injecting valve or a throttle body and this deviation cannot be separated from the deviation due to the change of the air density, the deviation corresponding to the change of the air density, which can be inherently indiscriminately learned, should be learned for respective areas of the driving state of the engine, and in the case where the automobile abruptly ascends to higher altitude, learning for the respective areas is impossible and learning is not substantially advanced.
The premise of learning is that the air/fuel ratio feedback control is carried out. However, in the conventional techniques, the air/fuel ratio feedback control is carried out only in the low-engine speed, low-load driving region (inclusive of the medium-engine speed, medium-load driving region) set as the air/fuel ratio feedback control region. (However, the air/fuel ratio feedback control is not carried out in the deceleration driving or when the temperature of the exhaust gas is low as is above set forth). The reason is that if the feedback control to the theoretical air/fuel ratio, that is, the aimed air/fuel ratio, is carried out in the high-rotation or high-load region, there is a risk of seizure of the engine or burning of the catalyst by elevation of the temperature, and therefore, in this region, the feedback correction coefficient is clamped and a rich output air/fuel ratio is separately obtained to prevent seizure of the engine.
Accordingly, when the automobile ascends a mountain, the driving is performed mainly in the high-load region and the air/fuel ratio feedback control is hardly performed, and hence, learning is not substantially carried out. This is another reason why the deviation corresponding to the change of the air density cannot be promptly learned.